Many current spacecraft are required during missions to autonomously maintain a state that is power, thermal and momentum safe indefinitely in the presence of external disturbances. Indefinite power and thermal safety can be satisfied for a deployed spacecraft by placing it in a sun hold attitude. However, maintaining the spacecraft in a momentum safe state requires some momentum unloading technique.
Many attitude control strategies utilize a reaction wheel system having a limited capacity to store accumulated momentum resulting from solar pressure on a spacecraft. For symmetric spacecraft for which the solar torque is small or for spacecraft which are not required to remain in sun hold for extended periods of time, the accumulation of momentum is not a major concern and therefore, the accumulated momentum will generally not exceed the storage capacity of these reaction wheel systems.
Conversely, for asymmetric spacecraft, such as any spacecraft where instrument field of view requirements drive the spacecraft layout, solar torque can be very large relative to the capacity of the reaction wheel system to store the accumulated momentum. Further, for spacecraft which are required to operate autonomously in sun hold for long periods of time, momentum accumulation is a concern regardless of how well the spacecraft is balanced. In these cases, the accumulated momentum would quickly exceed the capacity of the system. However, it is undesirable to use thrusters to perform frequent momentum unloading operations as this increases operational risk as well as wasting valuable fuel.
Accordingly, a momentum unloading technique for an asymmetric spacecraft which is required to operate autonomously in sun hold for long periods of time without the need for the firing of thrusters would be advantageous.